Process for forming reactive layers whose thickness is independent of time

ABSTRACT

A method for forming reactive layers on materials where the final thickness of the reactive layer is independent of time after an initial time period in which the final thickness is attained. Films are grown by a process, such as oxidation, and are removed simultaneously by a process, such as sputtering. The parameters of the growth process and removal process are initially fixed so that at a desired final thickness, the growth rate is equal to the removal rate. When this steady-state equilibrium is reached, the thickness of the reactive layer will remain constant, independent of time. The reactive layer can be grown to the desired thickness by this method, or a thicker layer can be reduced to a lesser final thickness by this method. Very small (less than 50A) films of excellent quality can be made.

Greiner States Metal [191 NOV. 19, 1974 [75] Inventor: James H. Greiner,Millwood, NY.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: Mar. 19, 1971 [21] Appl. No.: 125,993

[52] US. Cl 204/192, 117/106 A, 117/107, 117/201, 204/298 [51] Int. Cl.C236 13/00, C23C 15/00 [58] Field of Search 204/ 192 [56] ReferencesCited UNITED STATES PATENTS 3,021,271 2/1962 Wehner 204/192 3,324,0196/1967 Laegried et a1.... 204/192 3,325,393 6/1967 Darrow et al 204/1923,394,066 7/1968 Miles 204/192 3,400,066 9/1968 Caswell et al 204/1923,479,269 11/1969 Byrnes et al 204/192 3,640,812 2/1972 VoSSen et a1.204/192 OTHER PUBLICATIONS Wernick et a1., Surface Treatment ofAluminum, pg. 345-348, 1964.

OHanlon, Plasma Anodization of Metals and Semi- Schr'oen, Physics ofPreparation of Josephson Barriers, Jour. of Applied Physics, Vol. 39, p.2671 (May 1968).

Miles et al., The Formation of Metal Oxide Films Using Gaseous and SolidElectrolytes, J.E.C.S., Vol. 110, p. 1240 (Dec. 1963).

Worledge et a1., Controlled Oxidation of Tantalum and Aluminum in aRadio-Frequency-Exited Glow Discharge, British Journal of AppliedPhysics, Vol. 18, p. 1337 (1967).

Primary Examiner-John H. Mack Assistant ExaminerWayne A. LangelAttorney, Agent, or Firm-Jackson E. Stanland 5 7] ABSTRACT A method forforming reactive layers on materials where the final thickness of thereactive layer is independent of time after an initial time period inwhich the final thickness is attained. Films are grown by a process,such as oxidation, and are removed simultaneously by a process, such assputtering. The parameters of the growth process and removal process areinitially fixed so that at a desired final thickness, the growth rate isequal to the removal rate. When this steady-state equilibrium isreached, the thickness of the reactive layer will remain constant,independent of time. The reactive layer can be grown to the desiredthickness by this method, or a thicker layer can be reduced to a lesserfinal thickness by this method. Very small (less than 50A) films ofexcellent quality can be made.

27 Claims, 10 Drawing Figures PATEN um I 91974 THICKNESS SHEET 1 BF 2IIIATER FIG. 2A

OXIDATION TIME FIG. 2C

OXIDATION RATE THICKNESS x PUMP 25 FIG. 2B

SPUTTERING TIME INVENTOR JAMES H. GREINER AGENT PATENTEL NS! 1 9 1974FIG. 3A

SUBSTRATE h TIME T FIG. 33

T Tm h+A SUBSTRATE TIME T2 OXIDATION AND SPUTTERTNG RATES EQUAL FIG. 3C

SUBSTRATE TIME T OXIDATION AND SPUTTERING RATES REMAIN CONSTANT ANDEQUAL SHEET 2 OF 2 FIG.4A

OXIDE TIME T PROCESS FOR FORMING REACTIVE LAYERS WHOSE THICKNESS ISINDEPENDENT OF TIME BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to a method for forming reactive layersof desired thicknesses, and specifically to a method for formingreactive layers having reproducibly accurate final thicknesses whichremain constant even if the method is continued further.

2. Description of the Prior Art Reactive layers, such as oxides,nitrides, sulfides, etc. are useful in the production of many devices.For instance, Josephson junction devices use very thin (less than 50A)reactive layers as tunnel barriers between two superconductingelectrodes. Generally, the base electrode is oxidized to form the tunnelbarrier. Another device using reactive layers is a field effect devicehaving an oxide insulator below its gate electrode.

Reactive layers are defined generally as layers formed on base materialsand are combinations of the base material and other reactive materials.For instance, if lead is oxidized by oxygen species located adjacent tothe lead substrate, a reactive layer of lead oxide will be formed on thelead substrate.

Many processes have been used for depositing reactive layers on metalsand semiconductors. A review paper treating these processes if J. F.OHanlon, Journal of Vacuum Science and Technology, Vol. 7, No. 2, Page330 (1969). This paper notes that plasma anodization has been used toproduce oxide films where the samples to be anodized are located betweenthe cathode and the anode. Both RF and DC discharges have been used.

The following references discuss many aspects of production of reactivefilms, some of which are very thin for particular use in Josephsonjunctions. These references describe thermal oxidation, evaporation,sputtering. and anodization.

' W. Schroen. Journal of Applied Physics, Vol. 39, No.

6, May 1968, Page 2,671.

J. L. Miles and P. H. Smith, Journal of the Electrochemical Society,Dec. I963, Page 1,240.

L; D. Locker and L. P. Skolnick, Applied Physics Letters, Vol. 12, No.ll, 1 June 1968. Page 396.

G. A. Jennings and W. McNeill, Applied Physics Letters, Vol. 12, No. 2,Jan. 1968, Page 25.

P. L. Worledge and D. White, British Journal of Applied Physics, Vol.18, 1967, Page 1,337.

R. I. Nazarova, Russian Journal of Physiochemistry,

Vol. 36, No. 5. May 1962, Page 522.

M. Scharfe, The Gaseous Anodization of Tantalum. Thesis for MSEE,University of Minnesota, July 1966.

In the production of reactive layers, it is important that the thicknessof the formed layers be reproducibly obtained over many cycles of theprocess. This is especially true if the films to be produced are verythin, as is the case with Josephson devices. Since the tunneling currentin Josephson devices varies exponentially with the tunnel barrierthickness. small variations in thickness from one device to another canlead to serious problems when arrays are to be fabricated.

Another problem that occurs frequently in prior art methods forformation of reactive layers involves impurity incorporation in thegrowing film. Often. the substrate on which the reactive layer is to beproduced is placed in the glow discharge between a cathode and an anode.Sputtering from the cathode 'occurs and often the cathode material findsits way to the target, even if shieldingis used.

It is also important that the quality of the reactive film be good. Itshould be a very dense film having no pinholes or impurities thereinwhich may cause problems such as electrical shorting in devicesincorporating these films. In prior processes, ultra clean systems arerequired and the quality of the grown films was difficult to evaluateover a period of deposition ruins.

Prior'methods for forming reactive layers do not provide good control ofthe final thickness of the layer. Generally the process is stopped aftera certain amount of time, knowing that the thickness of the film wouldhave an approximate value after a certain amount of time had passed.However, formation of very thin films (less than 50A) is difficult whenthe only control over thickness is the amount of time during which theprocess is allowed to continue. For instance, water vapor may be presentin the system and may contribute additional oxygen ions. If an oxidefilm is being formed, the initial oxidation rate may be very high, sothat the film achieves a thickness greater than that desired before theprocess can be terminated. In this situation, the

quality of the film is apt to be inferior, in addition to its being toothick. The prior art methods do not have good thickness control, sincethe primary technique for obtaining thickness control is to slow thegrowth rate and then turn off the process after a period of timecalculated to achieve a desired thickness.

Accordingly, it is a primary object of this invention to provide aprocess for growing reactive layers which is time independent.

Another object of this invention is to provide an improved process forgrowth of reactive layers, in which layers of any desired finalthickness can be grown.

Still another object of this invention is to provide a process forgrowth of reactive layers in which the thickness ofthe grown layers isreproducibly constant during each deposition.

A further object of this invention is to provide a process for formationof reactive layers, where the thickness of the layers is uniformthroughout the layer.

A still further object of this invention is to provide a timeindependent process for formation of reactive layers, even if theinitial thickness of the reactive layer is greater than that desired.

Another object of this invention is to provide an improved process forformation of reactive layers which is time independent and whichproduces high quality reactive layers.

Still another object of this invention is to provide an improved processfor formation of reactive layers which is time independent and easilycontrolled to provide reactive layers of any desired thickness,regardless of the cleanliness of the process environment system and thesubstrate used.

SUMMARY OF THE INVENTION This method forms reactive layers on anysubstrates, such as semiconductors or metals. The layers formed includeoxides, nitrides. sulfides, etc. A reactive species is brought intocontact with the substrate and the reac tive layer is formed thereon.While the layer is forming, another process occurs simultaneously whichremoves the layer. After a period of time, a steady state equilibrium isproduced at which the growth rate is equal to the removal rate. At thisthickness, the reactive layers will maintain a constant thickness,independent of time.

In theparticular case of an oxide film, a low power RF oxygen dischargecan be used to form the oxide layer. A substrate is affixed to thecathode in the discharge and consequently will be both oxidized andsputtered during the process. If the oxidizing rate is initially largerthan the sputtering rate, an oxide film will form. The oxide ratedecreases as the oxide thickness increases because it is intrinisicallya diffusion limited process. However, the oxide sputtering rate isindependent of the thickness of the bombarded oxide. Therefore, if theoxidation rate is initially higher than the sputtering rate the oxidelayer thickness will increase until the rate of oxidation becomes equalto the sputtering rate, at which time a steady state oxide thickness isattained.

Conversely, if an oxide film is present and the sputtering rate isinitially higher than the oxidation rate, the oxide thickness decreasesuntil the oxidation and sputtering rates become equal, or until theoxide is removed. The final thickness of the oxide layer will dependupon the equalization of the sputtering rate and the oxidation rate,both of which can be initially established to yield the desired finalthickness at equilibrium.

As another alternative, an oxide layer can be reduced by a species suchas hydrogen, which is present in the glow discharge. This will lead toproduction of a pure base material having a desired thicknessestablished by the system parameters. For instance, a film of lead oxidecan be reduced by bombardment of hydrogen ions, to produce a pure leadlayer on the lead oxide, the pure lead layer having a constant finalthickness determined by the system parameters.

This process is particularly attractive for formation of very thin filmsof constant thickness. Films less than 100A can easily be producedmerely by setting the system parameters, which include l the temperaturethe substrate. (2) the RF power, (3) the pressure of the reactivespecies, and (4) the composition'of the gaseous discharge. Some of theseparameters affect the removal process (sputtering), while others have agreater effect on the growth process (oxidation). These can be setinitially, and no dependency on other factors, such as impurity levels,is present. Once a steady state equilibrium is reached, the filmthickness will remain constant and the process can be run for any lengthof time in order to provide high purity films having constant thickness.

In the preferred system, the substrates are placed on the cathode and anRF glow discharge is established between the cathode and an anode. Thereactive species is introduced into the chamber containing the cathodeand the anode. and a discharge is maintained by application of RF powerbetween the cathode and the anode. The system parameters mentioned aboveare adjusted so that, at a desired final thickness, the deposition rateand the removal rate will be equal. Since the sample is placed on thecathode, deposition of cathode material on the sample can be eliminated,which is not the case if the substrate were placed in the glow dischagebetween the cathode and the anode. ln addition to the contaminationproblem, systems in which the substrate is placed between the cathodeand the anode produce films of varying thickness, since some sputteringmay occur between the cathode and the substrate. In those systems, thefinal thickness is generally limited by the geometry of the system.Consequently, a process in which the substrate is placed on the cathodeavoids these problems and yields films of constant thickness havingminimal impurity contamination, and in which there is no deposit otherthan that from the intended source.

Since the solid state equilibrium is reached quickly in this process,good quality films are produced. This is a high energy process in whichions are propelled at the target, in contrast to the production ofreactive layers by thermal processes. Further, since the reactive filmis being removed at the same time it is formed, new reactive material isbeing constantly established. This means that any impure initial layerwill be removed and, as the system becomes more clean, the quality ofthe grown layer will improve.

This method has another advantage in that the composition of thereactive film can be tailored by varying the composition of thedischarge ambient. Regardless of the thickness required, chemicallydifferent reactive layers may be produced by incorporating differentamounts of the reactive species in the gaseous ambient.

Because the substrate is placed on the cathode, there is nothing toperturb the discharge existing between the cathode and the anode. Thisleads to a process having greater control, resulting in uniform films ofreproducible thickness. Since an equilibrium process is described,constant thickness independent of time can be produced continually. Formanufacture of large arrays of devices and for reactive layer formationson a continuing basis, no danger exists that the thickness will changefrom one system to another and from one formation run to another. Thusthe process is especially attractive for use in production lines wherecountless numbers of devices using reactive layers are produced.

Another advantage in the present method is that initial cleaning stepsare not required. ln conventional processes, a long initial dischargetime is required to clean the system before deposition onto thesubstrate. With the present system impurities such as H O do notinfluence the quality of the final reactive layer. If the layerinitially grows too thick due to the impurities, it will settle back toits desired final thickness when the rate of growth is equal to the rateof removal. Also, the initially produced impure film will be removedduring the formation process, with the result that the film producedafter steady state equilibrium is achieved will be very pure.

Although DC discharges can be used, RF discharges are preferred forgrowth of the reactive layers. Discharges can be sustained at lowerpressures when RF power is used, as opposed to the higher pressuresneeded to sustain the discharge when DC power is used. This makes itmore easy to remove adsorbed particles on the reactive layer beforedeposition of a subsequent layer. For instance, if an oxide layer isproduced on a base electrode and it is desired to produce a counterelectrode on this oxide layer, removal of adsorbed oxygen atoms from thereactive layer before deposition of the counter electrode is more simpleif an The prior art does not teach that a growth process can be balancedwith a removal process to form reactive layers having any desiredthickness, where the final thickness is independent of time. While theart'does suggest that terminal thicknesses can be the result of variousprocesses, nowhere is it suggested that any desired thickness can beachieved by initially selecting system parameters to force competingprocesses to be equal at that thickness. It has also been known thatprocesses such as oxidation produce films of limiting thickness, in thiscase due to the diffusion limited nature of the oxidation process.However, it is believed that applicant is the first to teach thatcompeting processes should be used to produce films of any desiredthickness, and that it makes no difference whether the film is grownfrom zero thickness or is reduced from a thickness greater than thatdesired. The process enables reproducible formation of films having anydesired thickness, totally independent of the time of formation oncesteady state equilibrium is achieved. Of course, the time period forachieving steady state equilibrium will vary depending upon thethickness desired, and the initial setting of the system parameters.

While sputtering and deposition within the sputtering apparatus is aparticularly useful embodiment, other growth and removal processes canbe used according to this invention. For instance, the removal processcan be achieved by an apparatus separate from the growth process. Anexample of this is the use of an ion beam to etch away the depositingreactive layer. Another combination of growth and removal processes maybe a liquid anodization growth process (for example to oxidize asubstrate), in, which the electrolyte dissolves some of the grown layerinto solution, thereby removing it. Consequently, it is apparent thatthe growth and removal processes can be separate, independentlyadjustable processes using the same or separate means for performingthose processes. Also, the rates of the growth and removal processes canbe varied at any time during the formation of the reactive layer or theycan initially be set to reach steady state equilibrium at some thicknessof the reactive layer without further adjustment.

Theforegoing and other objects, features and advantages of the inventionwill be apparent from the following more particular description of thepreferred embodiments of the invention as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of an RFsputtering apparatus suitable for carrying out the present method.

FIG. 2A is a plot of thickness of the formed reactive layer as afunction of time for a growth process, in this case oxidation.

FIG. 2B is a plot of the thickness ofthe reactive layer as a function oftime for a removal process. in this case sputtering.

FIG. 2C is a plot of the rates of the growth process (oxidation) and theremoval process (sputtering) as a function of time.

FIGs. 3A. 3B. and 3C illustrate the growth of a reactive layer on asubstrate with time. the reactive layer being conveniently shown as anoxide.

FIGS. 4A, 4B and 4C show the production of a pure layer from a reactivelayer, where the reactive layer is conveniently shown as an oxide andthe pure layer is conveniently shown as a metal forming a constituent ofthe oxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS This method employssimultaneously occuring growth processes and removal processes in orderto obtain good quality, reproducibly uniform reactive layers on varioussubstrates. In particular, the method will be described by describingthe formation of a controllable oxide layer on a substrate. It should beunderstood that other reactive layers, such as nitrides, sulfides, etc.can be formed on a base material (substrate). Whatever the speciescomprising the reactive layer, growth of the layer will occur at thesame time that a portion of the newly grown layer is removed. In thisway, the quality of the material grown will be good. Because the growthprocess and the removal process are independently controllable, therates of each process can be made equal at any thickness.

For the specific case of an oxide reactive layer, a low power RF oxygendischarge can be used to form the reactive layer. The underlyingsubstrate is fixed to the cathode in the discharge and is both oxidizedand sputtered. In this case the growth process is oxidation while theremoval process is sputtering. If the oxidizing rate is initially largerthan the sputtering rate, an oxide film will form. The oxidizing ratedecreases as the oxide thickness increases because oxidation isintrinsically a diffusion-limited process. However, the oxide sputteringrate is independent of the thickness of the bombarded oxide. Therefore,if the oxidation 'rate is initially higher than the sputtering rate, theoxide thickness will increase until the rate of oxidation becomes equalto the rate of sputtering, at which time a steady state oxide thicknessis obtained. This thickness will remain constant independent of timeeven if the method is continued.

Conversely, if an'oxide film is initially present and the sputteringrate is initially larger than the oxidation rate, the oxide thicknesswill decrease until the oxidation rate and the sputtering rate becomeequal, at which time a steady-state equilibrium will be achieved. Withan applied RF voltage, substantial equality of rates means that over afull cycle of applied RF voltage the oxidation and sputtering cancel oneanother. It should be understood that within each half cycle of appliedRF power, either oxidation or sputtering may dominate;

however, after a full cycle, the amount grown is the same as the amountremoved. Using this method, very small oxide layers 20-50A in thicknesshave been formed. Of course, the final thickness of the film can beadjusted by appropriately varying the system parameters. Theseparameters include the oxygen pressure, the RF power, the composition ofthe discharge, and the substrate temperature. The rate of sputtering andthe 'rate of oxidation are independently controllable using theseparameters; therefore. any desired final thickness can be achieved.

FIG. 1 shows an apparatus used for oxidation and sputtering to formreactive layers. This conventional aputtering apparatus comprises aglass chamber 10 having metal top and bottom plates 11A and 11B. RFcathode I2 is held by collar 13 which insulates it from top plate 11A.Cathode 12 is water-cooled, having an inlet pipe 14 and an outlet pipe16. The cathode is connected to an RF voltage source which provides anRF discharge at a power density of 0.03 to 2 watts/cm? The cathode issurrounded by a grounded shield 18 which protects against unwantedbombardment of the cathode. The substrates 19, onto which the reactivelayer is to be formed, are placed in a substrate holder 20 which isfastened to the cathode 12 by set screws 22. In a typical case, thecathode is copper and the substrate holder is aluminum, although othermaterials are often used.

Top and bottom metal plates 11A, 11B are grounded and serve as theanode. Connected into the chamber 10 by a valve 24 is a vacuum systemcomprising a freontrapped oil defusion pump 25 for providing the desiredvacuum. Also connected into the system is a manually controlled leakvalve 26 for introduction of the reactive gas species. Valve 26 isconnected to a source 27 of reactive gas species (and other gases) andthe amount of any gas introduced into chamber 10 is monitored. Ifdesired, the cathode can be surrounded by an aluminum ring which is usedfor cleaning during an initial DC discharge.

Also included in chamber 10 is a source 28 of material which can bedeposited onto substrates 19, or onto formed reactive layers. Insulatingmaterial 30 provides electrical isolation from base plate 11B. Locatedabove source 28 is a movable shutter 32 connected to a turning knob 34.Shutter 32 prevents sputtering from source 28 when located above source28.

The apparatus described is a conventional RF sputtering apparatus;however, the substrates are placed on the cathode rather than beinglocated between the cathode and the anode somewhere in the discharge. Ina typical operation. the substrates are initially cleaned bysputter-etching. for instance at a power density of 0.8 watts/cm at anargon pressure of 4 X 10 Torr. The argon leak into chamber 10 isbalanced by the throttled diffusion pump 25 in order to maintainconstant pressure. Typical cleaning times used to remove any residueoxides, photoresistive particles, or other contaminants vary fromapproximately lminutes.

Immediately after the sputter-etching cleaning steps, the argon inchamber is removed and is replaced by oxygen or by an argon-oxygenmixture. The reactive oxide layer is formed on substrate 19 bymaintaining an RF discharge at a power density of approximately 0.030.lwatts/cm for l0 to minutes. In this particular example. a reactive oxidelayer having a thickness less than 50A is produced.

The argon in the gas mixture during formation of the reactive layer isionized and bombards the cathode sputtering away portions of the oxideformed there by the oxygen gas species present in the chamber. If noargon is present. the oxygen gas species themselves will perform thesputtering function, but the sputtering rate will be less than thatwhich occurs when argon is also present in the discharge.

In order to account for the oxide growth. consideration must be given tothe sputtering and oxidation processes which occur at the cathode. Mostlikely, the sputtering occurs in the usual manner as a result ofpositive ion bombardment while the oxidation is thought to be dependenton the presence of negative oxygen ions. The oxygen discharge issustained by driving the cathode at a frequency of 13.56 MHz. Thisresults in the usual ion sheath that is visible as a dark space near thecathode surface. The cathode surface potential varies with time having apeak negative value nearly as large as the peak amplitude of the appliedpotential. Sputtering occurs when positive ions from the sheath aredrawn toward the cathode by the negative potential. It is likely thatthe negative ions for oxide growth are extracted from the plasma orcreated at the oxidegas interface by electron attachment. The negativebias resulting from the electron accumulation at the oxidegas interfacemay enhance oxidation by providing an additional driving force fordiffusion of cations through the oxide. In addition, high energyneutrals and implanted oxygen ions may be important in the sputteringand oxidation processes.

Although the sputtering and oxidation mechanisms have not beenidentified in detail, insight can be obtained regarding the dependenceof oxide thickness on process parameters by assuming that the sputteringand oxidation rates are additive. Accordingly, the rate of oxidethickness change is dx/dt dx/dt oxidation dx/dt sputter where x is theoxide thickness.

In thermal oxidation. the oxidation rate is generally found to be adecreasing function of oxide thickness. For very thin oxide films, adirect logarithmic growth with time is frequently found. Miles and Smith(referenced previously) have reported a logarithmic oxide growth withtime on vacuum deposited Al films exposed (without any external appliedpotential) to a dc oxygen glow discharge. If, in the present process,the oxidation is assumed to follow a logarithmic law, the oxidation ratemay be represented by the expression Ke' where K and x are oxidationparameters which depend on factors such as pressure and temperature. Inaddition, if the sputtering rate is assumed to be a constant (R) when anoxide is present. the Eq. (1) reduces to It is interesting to note fromEq. (2). that a balance between the sputtering and oxidation will occurwhen dx/d! 0. implying steady state type of saturation in contrast tothermal or dc glow discharge oxidation. The associated steady-stateoxide thickness is an x In (K/R) hereafter referred to as the limitingoxide thickness. From Eq. (3) it follows that it should be possible toobtain different limiting oxide thicknesses by varying the oxidation andsputtering parameters, e.g., oxygen pressure. RF input power andsubstrate temperature.

This is consistent with laboratory development in which the limitingoxide thickness was found to increase with oxygen pressure. indicatingthat the oxidation rate increases more rapidly with oxygen pressure thandoes the sputtering rate. The oxide thickness could be further adjustedby using an oxygen-argon gas mixture which increased the sputtering rateand decreased the oxidation rate as compared to percent oxygen. Thesputtering rate increases with RF power due to an increase in the iondensity and ion energy. Laboratory results indicate that the sputteringrate has a stronger dependence than the oxidation rate on the RF power.Since the energy of the bombarding ions is large compared to kT (k isthe Boltzman constant, and T is substrate temperature) little change insputtering rate occurs for increasing target temperature. However, theoxidation is a diffusion dependent process and is temperature dependent.

If Eq. (2) is integrated, the dependence of oxide thickness of time isobtained, namely,

where x,- is the initial oxide thickness. Eq. (4) reduces to (3) when eapproaches zero. It should be noted that the limiting oxide thicknessmight be less than, equal to, or greater than the initial oxidethickness depending on the amplitude of the initial oxidation rate ascompared to the sputtering rate.

An effective time constant, r /R can be estimated from Eq. (4). Fromthermal oxidation data and from oxidation in a dc oxygen glow discharge,x is estimated to be in the range of 1.5 to 3.5A. The sputtering rate isabout 0.lA/sec for a power density of 0.1 watts/cm for oxygen pressuresin the Torr. range. Hence, the effective time constant is estimated tobe about a minute.

The system parameters which can be varied to affect thesputtering/oxidation processes are (l) substrate temperature, (2) RFpower, (3) reactive gas species, (4) composition of the gaseousdischarge.

If the substrate temperature is varied, the oxidation rate is changedalthough the sputtering rate is not appreciably affected. Sinceoxidation is diffusion limited, increasing the substrate temperaturewill increase the oxidation rate. As the temperature decreases, theoxidation rate will also decrease.

Sputtering rate is dependent on RF power. As the voltage on the cathodeincreases, there will be a high efficiency of ionization of oxygen andthe sputtering rate will increase. While variation of the RF power may.

also vary oxidation rate, the primary change is to the sputtering rate;Since the voltage on the cathode determines the ion energy striking thecathode, this is a direct way to increase sputtering.

The reactive'gas pressure in the discharge (assume that it is oxygen)directly affects the growth process. As the pressure of oxygen in thedischarge increases, more oxygen ions will be produced per unit time sothe oxidation rate will increase at a constant RF power. However. it ispossible that sputtering will increase also since more oxygen ions arepresent. It has been found that the thickness of films increases whenthe reactive gas species pressure increases, which indicates that theoxidation (growth) process is primarily increased by varying thereactive gas species pressure.

Varying the composition of the discharge can vary either the oxidationor the sputtering rates, or both. For instance. if additional oxygen isadded to the discharge. the oxidation rate will increase. Conversely, ifargon is added to the discharge. the sputtering rate will increase.

FIGS. 2A-2C show the growth process, removal process, and the rate ofchange in these processes. In these figures, it is assumed that thereactive layer to be produced is an oxide layer, although it should beunderstood that other reactive layers can also be produced bycorresponding processes.

In FIG. 2A, the thickness of a reactive layer in the presence of anoxidation process is plotted against time. It is seen that a terminalthickness is eventually achieved, since oxidation is primarily adiffusion limited process.

In FIG. 2B, the thickness of a reactive film removed by a sputteringprocess is plotted against time. Since sputtering removes the atoms fromthe reactive layer linearly with time, the plot is a linearly decreasingcurve.

FIG. 2C shows a plot of the growth process rate and the removal processrate, for the growth process of FIG. 2A and the removal process of FIG.2B. The oxidation rate curve decreases exponentially with time,resulting in a substantially constant rate after a sufficient amount oftime has passed. The sputtering rate is constant since the curve of thesputtering process (FIG. 2B) is linear. At a time T the oxidation rateand the sputtering rate are equal. At this time, the reactive layer hasachieved its final thickness and from then on will maintain thisthickness independent of time. This cross-over point of the growth andremoval rate curves is determined by the desired final thickness. Aparticular amount of time T will be required to get to this cross-overpoint, but from then on the reactive layer will maintain a constantthickness with time. During the applied RF voltage cycle, growth of thereactive layer will predominate during one-half of each cycle, whilesputtering (removal) will predominate during the other one-half cycle.Consequently, the average change over a cycle is zero.

FIGS. 3A-3C demonstrate the production of a reactive layer by thisprocess. In FIG. 3A, a substrate having height h is shown at time T,.This substrate is placed in the apparatus of FIG. 1, andthe method asdescribed occurs. Assuming that the reactive layer is to be an oxide, anoxide having a terminal thickness X is produced after a time period (T-T at the end of which the oxidation rate is equal to the sputteringrate. The new thickness of the oxide/substrate is now (lH-A), due toexpansion when the substrate is oxidized.

At a later time T5, the oxide has the same final thickness X but theoverall thickness of the oxide and the underlying substrate is reducedto a value less than h. As the process continues from time T additionaloxide and substrate atoms are sputtered away although the oxidethickness remains constant, since the oxidation rate and the sputteringrate are equal, and remain constant in magnitude.

FIGS. 4A-4C show the effect of a reduction process in accordance withthe method of the present invention. In this process, an initial oxidelayer exists at time T,, as shown in FIG. 4A. It is desired to reduce aportion of this oxide layer to leave a layer of pure constituents of theoxide. In FIG. 4B, this is shown as a metal for illustration purposes.For instance, the oxide of FIG. 4A can be lead oxide which is to bereduced into an oxide layer and a pure lead layer.

If the oxide layer of FIG. 4A is a substrate in the apparatus of FIG. 1,and if the reactive gas is hydrogen, the incidence of hydrogen ions onthe substrate oxide will reduce the oxide, leaving a pure lead layer onthe lead oxide. Again. the rate of hydrogen deposition and the rate ofsputtering from the oxide surface is balanced to produce a pure metallayer of predetermined thickness which remains constant with time,'aftera steady state equilibrium has been reached at time T In FIG. 4C, thereduction process has continued, but the thickness of the metal layerremains X since the growth and removal processes are equal, and haveconstant magnitudes. However, the thickness of the oxidemetalcombination is less than h.

In the described method, the substrate can be any material includingmetals and semiconductors. The reactive layers can also be a variety ofmaterials including oxides, nitrides, sulfides, and semiconductors, etc.For instance, if the reactive gas species is H 5 and lead is thesubstrate, a lead sulfide layer of controlled thickness will be formed.Lead sulfide is a semiconductor material, so this illustrates an exampleof the production ofa controlled thickness of a semiconductor material.

The method is particularly useful in semiconductor technology and in thefabrication of Josephson junction devices. Josephson devices requirevery thin tunnel barriers having reproducibly uniform thicknesses, andthis process is particularly useful. By proper adjustment of the systemparameters, a controlled tunnel barrier of any thickness can beproduced. The same system can then be used to deposit a counterelectrode merely by introducing a source of material to be deposited onthe previously produced tunnel barrier. For example, a lead evaporationsource 28 can be incorporated in the chamber of FIG. 1. The substratecould be niobium 6,000A in thickness deposited by RF sputtering in argonat a pressure of 10 Torr. and at a rate of approximately 400A/minuteonto glass substrates held at 670K. During deposition, the substratesupport is grounded. To fabricate the tunnel barrier, the niobium filmsare affixed to an RF cathode l2 and the abovedescribed methodisundertaken to produce an oxide tunnel barrier. The lead counterelectrode is then evaporated onto the tunnel barrier.

As stated previously, the operation is not limited to metals or to anoxygen glow discharge. If other gas species or mixtures are used, (forinstance H N H CH etc. many other reactive layers can be made. Theformation of uniform. dense, short-free, reproducible reactive layers onbase materials is achieved in many technologies other than Josephsontunneling devices. For example, semiconductor technology, MOS, and FETstructures utilize reactive layers.

Controlled multilayer materials are also obtained by this process, sincea reactive gas species can be altered as the depositions proceed. lnaddition, compositional or different structural forms of variousreactive layers can be controlled by varying the reactive conditions,

such as temperature, pressure, gas species. ion energy, etc.

What has been described is an improved process for forming reactivelayers in which a growth process and a removal process occursimultaneously to offset one another. The rates for each of theseprocesses are independently adjustable and a steady-state equilibrium inwhich the rates are equal determines the final thickness. In aparticular embodiment. oxidation and RF sputtering provide aparticularly good working example. However. it should be understood thatthe principle of this invention includes methods where the growthprocess is not oxidation and the removal process is not sputtering, aswas explained previously.

Films of any kind can be made through the use of competing growth andremoval processes to provide films of reproducible final thickness. Thefilm material is deposited on a substrate by any known process (such asevaporation, sputtering, plating, etc.) and a portion of the depositedfilm is removed simultaneously by any suitable process (such as ionbeam, electron beam, sputtering, etc.). The final thickness of the filmwill be determined by conditions at which the rate of deposi tion isequal to the rate of removal. When the rates are held equal, the filmthickness will remain the same, even if further deposition and removaloccurs. Thus the dependence of film thickness on time, which is presentwhen only deposition is used to grow a film, is eliminated as a variablein this process.

What is claimed is:

l. A method for forming reactive layers of controlled thickness on basematerials, said reactive layers being combinations of the base materialand at least one constituent which reacts with said base material,comprising:

bringing said reactive constituent to said base material in asubstantially vacuum environment for reaction thereon to grow saidreactive layer at a first rate;

removing said reactive layer simultaneously with said growth, theremoval of said reactive layer occurring at a seond rate;

adjusting said first and second rates to be equal for a desired finalthickness of said reactive layer; continuing said growth and saidremoval at least until said first and second rates of growth and removalrespectively are equal, at which time a final thickness of said reactivelayer is achieved. .2. The method of claim 1, where said base materialis a metal, and said reactive constituent is oxygen.

3. The method of claim 1, where said base material is selected from thegroup consisting essentially of metals and'semiconductors, and saidreactive constituents are selected from the group consisting of oxygen,nitrogen, hydrogen, sulfur, H 8 and CH.,.

4. The method of claim 1, where said first rate is dependent on thethickness of said grown reactive layer and said second rate issubstantially independent of the thickness of said reactive layer, saidfirst and seond rates tending to equalize as the thickness of saidreactive layer changes.

5. The method of claim 1, wherein said reactive layer is initiallypresent on said base material, and said second rate is initially greaterthan said first rate, said rates then varying relatively until a finalthickness of said reactive layer is reached at which said first andsecond rates are equal.

6. The method of claim 1, where said reactive layer grows by a vapordeposition process.

7. A method of forming reactive layers on base material, said layershaving a thickness independent of time after a final thickness isachieved, said method comprising:

placing said base material in a system having a cathode and an anodelocated in a chamber in which a vacuum can be established, said basematerial having the electrical potential of said cathode; introducing areactive gas species into said chamber,

said species being combined with said base material to grow saidreactive layer thereon when a voltage is established between saidcathode and said anode to provide a gaseous plasma between said cathodeand said anode, said reactive layer being removed by the incidence ofgaseous particles from said plasma as said growth occurs; adjusting therate of growth of said layer and the rate of removal of said layer toestablish a steady state equilibrium at which said rates aresubstantially equal, at the final desired thickness of said reactivelayer. 8. The method of claim 7, where said base material is selectedfrom the group consisting of metals and semiconductors, and saidreactive species is selected rate and the removal rate varyingrelatively to one another as the thickness of said reactive layerchanges; adjusting said first and second rates to be equal at a desiredfinal thickness of said reactive layer; continuing said growth and saidremoval at least until said first rate is substantially equal to saidsecond rate. 19. The method of claim 18, where said reactive layerformation occurs by oxidation and the removal of said reactive layeroccurs by sputtering.

20. The method of claim 18, where said substrate is selected from thegroup'consisting of metals and semiconductors, and said ambient includesa plurality of from the group consisting of oxygen, nitrogen y l5gaseous species, at least one of said species reacting gen, sulfur, CH H3, and combinations thereof.

9. The method of claim 7, where said base material is a metal and saidreactive species is oxygen.

10. The method of claim 7, where a plurality of reactive species areintroduced into said chamber, thereby resulting in a reactive layer ofvariable composition.

11. The method of claim 7, where an inert gas is also present in saidgaseous plasma.

12. The method of claim 7, where said voltage is an RF voltage appliedbetween said cathode and said anode.

13. A method of producing films of controllable thickness on asubstrate, comprising the steps of:

depositing gaseous particles of film material on said substrate, saiddeposition occurring at a first rate;

removing said film material from said substrate during said deposition,said removal occurring at a second rate which is independentlyadjustable with respect to said first rate;

adjusting said first and second rates to provide steady stateequilibrium conditions at which said rates will be equal;

continuing said deposition and said removal at least until steady stateequilibrium is attained. 14. The method of claim 13, wherein said firstrate is dependent on the thickness of said deposited material.

15. The method of claim 13, including the further step of depositing alayer onto said film material after said steady state equilibrium isattained.

16. The method of claim 13, where said steady state equilibrium isattained at a film thickness less than about 100A.

17. The method of claim curs by a vapor process.

18. A method of forming reactive layers on substrates, comprising thesteps of:

placing said substrate in a vacuum environment housing a cathode and ananode, said substrate'being located on the surface of said cathode;

introducing a gaseous ambient between said cathode and said anode, saidambient containing a species which reacts with said substrate to formreactive layers thereon;

establishing a voltage between said cathode and said anode to create anenergetic plasma about said substrate whereby said species from saidplasma strikes said substrate to form a reactive layer thereon having afirst growth rate, said reactive layer also being removed at a secondrate by the incidence of particles from said plasma, the growth 13,where said removal ocwith said substrate to form said reactive layer.

21. The method of claim 18, where said gaseous ambient contains at leastone gaseous species selected from the group consisting of oxygen,nitrogen, sulfur, hydrogen, CH and H 5.

22. A method of forming a multi-layer structure comprising the steps of:

placing a substrate in a vacuum environment containing a cathode and ananode, and a source of a first material; coating said substrate withsaid first material to form a base layer;

introducing a gaseous ambient between said cathode and said anode, saidambient containing gaseous species which react with said base layer toform a reactive layer thereon;

producing an energetic plasma about said base layer by applying avoltage between said cathode and said anode, said base layer being atsaid cathode potential, said gaseous species in said plasma strikingsaid base layer and forming a reactive layer on said base layer at afirst rate of formation, said reactive layer being removed by theincidence of particles from said plasma, said removal occurring as saidreactive layer is being formed and at a second rate of removal; v

adjusting the relative rates of formation and removal to provide asteady state equilibrium at which said formation rate will besubstantially equal to said removal rate;

continuing said formation and said removal at least until said steadystate equilibrium is achieved, at which time said reactive layer willhave ,a desired final thickness;

coating said reactive layer with an electrically conductive material toform a counter layer thereon.

23. The method of=claim 22, wherein said base layer and said counterlayer are metals and said reactive layer is an oxide.

24. The method of claim 22, wherein said voltage is an RF potential andsaid substrate and base layer are located on said cathode.

25. The method of claim 22, where said reactive gas species includes atleast one of the group consisting of oxygen, nitrogen, hydrogen, sulfur,CH and H 8.

26. A method for forming. reactive layers of con-v trolled thickness onbase materials, said reactive layers being combinations of the basematerial and at least one constituent which reacts with said basematerial, comprising:

bringing said reactive constituent to said base material for reactionthereon to grow said reactive layer at a first rate;

removing said reactive layer by a sputtering process which occurssimultaneously with said growth, the removal of said reactive layeroccurring at a second rate;

adjusting said first and second rates to be equal for a desired finalthickness of said reactive layer;

continuing said growth and said removal at least until said first andsecond rates of growth and removal respectively are equal, at which timea final thickness of said reactive layer is achieved.

27. A method of producing films of controllable until said steady stateequilibrium is attained.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 3,849,276

DATED Nov. 19, 1974 INVENTOR(S) James H. Greiner It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 8, line 32, "A 1" should read ASL (aluminum) x/x x/x Column 8,line 37, "Ke should read Ke x/x Column 8, equation 2, should read dx/dtKe O R for x O.

Column 9, equation 4, should read Xi/Xo Rt/x x x ln [K/R (K/R e e for xO.

...Rt lx Rt /Xo Column 9, line 18, "e should read e Column 9, line 23,"t R" should read t x /R-.

o xo/ D o o H II Column 11, line 44, H N H CH should read H N H 8, CH

Bigncd and Sealed this eleventh Day of May 1976 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Atlvsring Officer ('nmmissimu'r oj'lannrsand Trademarks

1. A METHOD FOR FORMING REACTIVE LAYERS OF CONTROLLED THICKNESS ON BASEMATERIALS, SAID REACTIVE LAYERS BEING COMBINATION OF THE BASE MATERIALAND AT LEAST ONE CONSTITUENT WHICH REACTS WITH SAID BASE MATERIAL,COMPRISING: BRINGING SAID REACTIVE CONSTITUENT TO SAID BASE MATERIAL INA SUBSTANTIALLY VACUUM ENVIRONMENT FOR REACTION THEREON TO GROW SAIDREACTIVE LAYER AT A FIRST RATE; REMOVING SAID REACTIVE LAYERSIMULTANEOUSLY WITH SAID GROWTH, THE REMOVAL OF SAID REACTIVE LAYEROCCURING AT A SECOND RATE; ADJUSTING SAID FIRST AND SECOND RATES TO BEEQUAL FOR A DESIRED FINAL THICKNESS OF SAID REACTIVE LAYER; CONTINUINGSAID GROWTH AND SAID REMOVAL AT LEAST UNTIL SAID FIRST AND SECOND RATESOF GROWTH AND REMOVAL RESPECTIVELY ARE EQUAL, AT WHICH TIME A FINALTHICKNESS OF SAID REACTIVE LAYER IS ACHIEVED.
 2. The method of claim 1,where said base material is a metal, and said reactive constituent isoxygen.
 3. The method of claim 1, where said base material is selectedfrom the group consisting essentially of metals and semiconductors, andsaid reactive constituents are selected from the group consisting ofoxygen, nitrogen, hydrogen, sulfur, H2S , and CH4.
 4. The method ofclaim 1, where said first rate is dependent on the thickness of saidgrown reactive layer and said second rate is substantially independentof the thickness of said reactive layer, said first and seond ratestending to equalize as the thickness of said reactive layer changes. 5.The method of claim 1, wherein said reactive layer is initially presenton said base material, and said second rate is initially greater thansaid first rate, said rates then varying relatively until a finalthickness of said reactive layer is reached at which said first andsecond rates are equal.
 6. The method of claim 1, where said reactivelayer grows by a vapor deposition process.
 7. A method of formingreactive layers on base material, said layers having a thicknessindependent of time after a final thickness is achieved, said methodcomprising: placing said base material in a system having a cathode andan anode located in a chamber in which a vacuum can be established, saidbase material having the electrical potential of said cathode;introducing a reactive gas species into said chamber, said species beingcombined with said base material to grow said reactive layer thereonwhen a voltage is established between said cathode and said anode toprovide a gaseous plasma between said cathode and said anode, saidreactive layer being removed by the incidence of gaseous particles fromsaid plasma as said growth occurs; adjusting the rate of gRowth of saidlayer and the rate of removal of said layer to establish a steady stateequilibrium at which said rates are substantially equal, at the finaldesired thickness of said reactive layer.
 8. The method of claim 7,where said base material is selected from the group consisting of metalsand semiconductors, and said reactive species is selected from the groupconsisting of oxygen, nitrogen, hydrogen, sulfur, CH4, H2S, andcombinations thereof.
 9. The method of claim 7, where said base materialis a metal and said reactive species is oxygen.
 10. The method of claim7, where a plurality of reactive species are introduced into saidchamber, thereby resulting in a reactive layer of variable composition.11. The method of claim 7, where an inert gas is also present in saidgaseous plasma.
 12. The method of claim 7, where said voltage is an RFvoltage applied between said cathode and said anode.
 13. A method ofproducing films of controllable thickness on a substrate, comprising thesteps of: depositing gaseous particles of film material on saidsubstrate, said deposition occurring at a first rate; removing said filmmaterial from said substrate during said deposition, said removaloccurring at a second rate which is independently adjustable withrespect to said first rate; adjusting said first and second rates toprovide steady state equilibrium conditions at which said rates will beequal; continuing said deposition and said removal at least until steadystate equilibrium is attained.
 14. The method of claim 13, wherein saidfirst rate is dependent on the thickness of said deposited material. 15.The method of claim 13, including the further step of depositing a layeronto said film material after said steady state equilibrium is attained.16. The method of claim 13, where said steady state equilibrium isattained at a film thickness less than about 100A.
 17. The method ofclaim 13, where said removal occurs by a vapor process.
 18. A method offorming reactive layers on substrates, comprising the steps of: placingsaid substrate in a vacuum environment housing a cathode and an anode,said substrate being located on the surface of said cathode; introducinga gaseous ambient between said cathode and said anode, said ambientcontaining a species which reacts with said substrate to form reactivelayers thereon; establishing a voltage between said cathode and saidanode to create an energetic plasma about said substrate whereby saidspecies from said plasma strikes said substrate to form a reactive layerthereon having a first growth rate, said reactive layer also beingremoved at a second rate by the incidence of particles from said plasma,the growth rate and the removal rate varying relatively to one anotheras the thickness of said reactive layer changes; adjusting said firstand second rates to be equal at a desired final thickness of saidreactive layer; continuing said growth and said removal at least untilsaid first rate is substantially equal to said second rate.
 19. Themethod of claim 18, where said reactive layer formation occurs byoxidation and the removal of said reactive layer occurs by sputtering.20. The method of claim 18, where said substrate is selected from thegroup consisting of metals and semiconductors, and said ambient includesa plurality of gaseous species, at least one of said species reactingwith said substrate to form said reactive layer.
 21. The method of claim18, where said gaseous ambient contains at least one gaseous speciesselected from the group consisting of oxygen, nitrogen, sulfur,hydrogen, CH4and H2S.
 22. A method of forming a multi-layer structurecomprising the steps of: placing a substrate in a vacuum environmentcontaining a cathode and an anode, and a source of a first material;coating said substrate with said first material to form a base layer;introducing a gaseous ambieNt between said cathode and said anode, saidambient containing gaseous species which react with said base layer toform a reactive layer thereon; producing an energetic plasma about saidbase layer by applying a voltage between said cathode and said anode,said base layer being at said cathode potential, said gaseous species insaid plasma striking said base layer and forming a reactive layer onsaid base layer at a first rate of formation, said reactive layer beingremoved by the incidence of particles from said plasma, said removaloccurring as said reactive layer is being formed and at a second rate ofremoval; adjusting the relative rates of formation and removal toprovide a steady state equilibrium at which said formation rate will besubstantially equal to said removal rate; continuing said formation andsaid removal at least until said steady state equilibrium is achieved,at which time said reactive layer will have a desired final thickness;coating said reactive layer with an electrically conductive material toform a counter layer thereon.
 23. The method of claim 22, wherein saidbase layer and said counter layer are metals and said reactive layer isan oxide.
 24. The method of claim 22, wherein said voltage is an RFpotential and said substrate and base layer are located on said cathode.25. The method of claim 22, where said reactive gas species includes atleast one of the group consisting of oxygen, nitrogen, hydrogen, sulfur,CH4, and H2S.
 26. A method for forming reactive layers of controlledthickness on base materials, said reactive layers being combinations ofthe base material and at least one constituent which reacts with saidbase material, comprising: bringing said reactive constituent to saidbase material for reaction thereon to grow said reactive layer at afirst rate; removing said reactive layer by a sputtering process whichoccurs simultaneously with said growth, the removal of said reactivelayer occurring at a second rate; adjusting said first and second ratesto be equal for a desired final thickness of said reactive layer;continuing said growth and said removal at least until said first andsecond rates of growth and removal respectively are equal, at which timea final thickness of said reactive layer is achieved.
 27. A method ofproducing films of controllable thickness on a substrate, comprising thesteps of: depositing particles of film material by oxidation on saidsubstrate, said deposition occurring at a first rate; removing said filmmaterial from said substrate by sputtering during said deposition, saidremoval occurring at a second rate which is independently adjustablewith respect to said first rate; adjusting said first and second ratesto provide steady state equilibrium conditions at which said rates willbe equal; continuing said deposition and said removal at least untilsaid steady state equilibrium is attained.